The present invention relates to light splitters as well as an optical transmitter configuration with a light splitter.
The present invention is based on problems such as were recognized when employing X-cubes and will be explained in the following. But the findings can, in principle, be transferred to optical light splitters which, as will yet be explained, are used in connection with light of different polarization. Regarding this topic, reference can be made to A. Thelen, "Nonpolarizing interference films inside a glass cube", Appl. Optics, Vol. 15, No. 12, December 1976.
For example, in DE 40 33 842 a cuboid optical structural component composed of single prisms with dichroic layers is referred to as a "dichroic prism".
In the present document the expression EQU X-cube
is used for such a structural component. With respect to such X-cubes reference is made to U.S. Pat. Nos. A 2,737,076, 2,754,718, DE A 40 33 842 as well as to JP 7 109443, further to U.S. Pat. No. 5,098,183, and EP A 0 359 461. Furthermore, reference is made to WO98/20383 by the same applicant.
Definitions
The following definitions are used:
Light, visible light: light with maximum energy in the spectral range 380 nm-720 nm PA1 red light: light with maximum energy in the spectral range 580 nm-720 nm, in particular in the spectral range 600 nm-680 nm PA1 green light: light with maximum energy in the spectral range 490 nm-605 nm, in particular in the spectral range 500 nm-600 nm PA1 blue light: light with maximum energy in the spectral range 380 nm-510 nm, in particular in the spectral range 420 nm-500 nm PA1 yellow light: light with maximum energy in the spectral range 475 nm-605 nm, in particular at 582.+-.3 nm PA1 white light: light with red, blue and green light components PA1 transparent: negligible absorption in the spectral range 380 nm-720 nm PA1 cube: spatial shape formed by identical rectangles pairwise opposing each other in parallel PA1 I.sub.Bout.sup.P : intensity of the P-polarized blue light reflected back by coating system 5, PA1 I.sub.in.sup.S : intensity of the S-polarized blue light incident on coating system 5, PA1 R.sub.RB.sup.S : the reflection of the blue-selective coating system 5 for S-polarized blue light, PA1 R.sub.RB.sup.P : the reflection of the blue-selective coating system 5 for P-polarized blue light. PA1 T.sub.RR.sup.S : the transmission of the red-selective coating system 7 for S-polarized light PA1 R.sub.RLVB : the reflection of the light valve PA1 T.sub.RR.sup.P : the transmission of the red-selective coating system 7 for P-polarized light. PA1 To summarize: thus a formulation according to the invention of the solution of the above task is the specific selection of materials of the described coating systems.
First, the fundamental effect of an X-cube will be explained with reference to FIG. 1. Optic light splitters of this type are employed primarily in projectors in order to separate white light into red, green and blue light or to recombine the latter into white light. According to FIG. 1, an X-cube comprises four single prisms 2a-2d. The prisms commonly comprise BK7 glass. In cross section they form right-angled isosceles triangles with an angle of 90.degree.. The length of the hypotenuse is, for example, between 5 mm and 50 mm, preferably 20-30 mm. Between the two prism pairs 2a and 2b, on the one hand, and 2d and 2c, on the other, a spectrally selectively reflecting and transmitting coating system 5 is embedded, which largely reflects blue light but largely transmits green and red light.
Between the two prism pairs 2a and 2d, on the one hand, and 2b and 2c, on the other hand, a further spectrally selectively reflecting and transmitting coating system 7 is embedded which largely reflects red light, however largely transmits green and blue light.
Consequently, on the X-cube three channels result for red, green and blue light, K.sub.R, K.sub.G, K.sub.B and one channel K.sub.R+B+G for white light. On each of the coating systems 5, 7, between the addressed prism pairs, reflection at 45.degree. of the incident light, thus colored light, takes place. The hypotenuse faces of the prisms 2 can be coated with an antireflection coating system.
Such X-cubes are mainly used today in projection apparatus in order to recombine red (R), blue (B), and green (G) light, each of which is supplied via light valves, in particular LCD light valves, operating in transmission to the associated channels K.sub.R, K.sub.B, K.sub.G in channel K.sub.R+B+G into white light. This is indicated in FIG. 1 in dashed lines. Light valves are therein image-forming elements comprising a multiplicity of individually driven pixels. The number of pixels therein yields the resolution according to EVGA, SGA, EGA, or XGA standards, etc.
Due to the printed conductor and the driving electronics a lower limit of the pixel size exists in the case of such light valves operating in transmission and it is only with difficulty possible to attain sizes below this limit. When decreasing the pixel size, furthermore, the optical aperture per pixel decreases.
This restriction does not apply in light valves which do not work in transmission but rather, as shown in solid lines using light valves LCD in FIG. 1, operate in reflection and therein rotate the plane of polarization of the reflected light by 90.degree..
The use of such light valves operating in reflection has been hindered until today by problems which will be explained later. In FIG. 2 the conditions are shown which obtain when replacing conventional light valves, which, according to FIG. 1, are LCD valves operating in transmission, by light valves RLV, which are reflective light valves operating in reflection. If, to the configuration according to FIG. 1, a light valve RLV operating in reflection is connected according to FIG. 2, for example, reflected S-polarized (direction of oscillation of the E field) blue light B reflected on coating system 5 of the X-cube according to FIG. 1, is converted on the light valve RLV into P-polarized blue light and reflected back onto the coating system 5 and, again, reflected by the latter. On one and the same coating system 5, according to FIG. 2, and, analogously, for red light on system 7, reflections of light of identical spectra but different polarizations occur.
Spectrally selectively reflecting and transmitting coating systems, such as are used in said X-cubes but also in other light splitters for color-selective effects, are conventionally produced by means of dielectric multicoating systems. These comprise each at least one layer of a material with lower refractive index and one layer of a material with higher refractive index. For example, as the material with lower refractive index SiO.sub.2 is conventionally used, with a refractive index of 1.46. As the material with higher refractive index, for example, TiO.sub.2 is used today with a refractive index of 2.4 or Ta.sub.2 O.sub.5 with a refractive index of 2.1.
In FIG. 3 the reflection of S-polarized blue light on a color-selective coating system comprising SiO.sub.2 /TiO.sub.2 is shown as well as that of P-polarized blue light on the same coating system. Both measurements took place at an angle of light incidence of 45.degree., as depicted in FIG. 2.
In FIG. 4 is shown on a coating system again composed of SiO.sub.2 /TiO.sub.2 layers and selectively reflecting red light R, the reflection behavior of S-polarized and of P-polarized red light. The measurements of FIGS. 3, 4 were carried out on an X-cube with BK7 glass as the base body material wherein the listed color-selective coating systems 5, 7 or FIG. 2 were embedded.
In FIGS. 3 and 4 is evident that, on the one hand, in both cases the reflection of P-polarized light is significantly less than that of S-polarized light, quite pronounced on the red-selective coating system, and that further a marked edge shift--polarization shift--of the reflected spectra takes place. For example, with selective reflection of blue light the 50% reflection points for S- and P-polarization are spaced over 70 nm apart, corresponding to .DELTA..sub.B.
If in FIG. 2 the represented path of rays is considered, without taking into account the second color-selective coating system 7 provided on the X-cube, thus only the reflection on one coating system, namely the coating system 5 for blue light, one obtains EQU I'.sub.Bout.sup.p =I.sub.in.sup.S.multidot.R.sub.RB.sup.S.multidot.R.sub.RB.sup.P (1)
where
Starting from the reflection behavior shown in FIGS. 3 and 4 for blue light B on coating system 5 according to FIG. 2 and, analogously, for red light R on coating system 7, taking into consideration the particular transmitting coating systems, thus system 7 for blue light B or system 5 for red light R, respectively, intensity spectra for I.sub.Bout.sup.P or for I.sub.Rout.sup.P are obtained as shown in FIG. 5 or FIG. 6, respectively. Taking into consideration the stated transmissions, as well as according to FIG. 2 the reflection behavior of a potentially added light valve RLV operating in reflection, for blue light is obtained: EQU I.sub.Bout.sup.P =I.sub.in.sup.S.multidot.T.sub.RR.sup.S.multidot.R.sub.RB.sup. S.multidot.R.sub.RLVB.multidot.R.sub.RB.sup.P.multidot.T.sub.RR.sup.P (2)
Therein denote further
The expression for red light is obtained analogously.
The differing reflection properties of the selective red or blue coating systems with respect to S- and P-polarization lead to serious consequences:
The light from the spectral ranges .DELTA..sub.B or .DELTA..sub.R between the S- and P-reflection spectra according to FIGS. 3 and 4 is not output but passes correspondingly the blue or red reflector and remains as scattered light in the system, according to FIG. 2 in the X-cube.
In FIG. 7 the spectrum of the scattered light in the X-cube is shown with the stated intermediate spectral ranges .DELTA..sub.B and .DELTA..sub.R according to FIGS. 3 and 4 entered.
It is evident that a large quantity of scattered light remains in the system. Further, as evident in FIGS. 5 and 6, the total transmission in the red spectral range as well as also in the blue is insufficient, i.e. substantially less than it would be, in view of FIG. 2, if on both color-selective coating systems 5 and 7 only S-polarized light were to be reflected.